This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-200130, filed Jun. 30, 2000; and No. 2001-144730, filed May 15, 2001, the entire contents of both of which are incorporated herein by reference.
The present invention relates to a power semiconductor switching element and, more particularly, to a semiconductor element having a low ON resistance.
Recently, power MOSFETs (power MOSFETs) have been widely used for power supplies in vehicles, power supplies for computer equipment, motor control power supplies, and the like. For these power supplies, importance is placed on efficiency and downsizing.
In switching power supplies that have been widely used, since power MOSFETs also serve as conventional diodes (synchronous rectification), the characteristics of power MOSFETs are very important. Two characteristics, ON resistance and switching speed, are especially important. As the ON resistance decreases, the energy consumed by a power MOSFET while a current flows decreases, and hence the efficiency of the power supply increases. As the switching speed increases, the switching frequency can be increased. This makes it possible to reduce the size of a magnetic circuit, e.g., a transformer. Therefore, the power supply can be reduced in size, and the efficiency of the magnetic circuit can be increased.
FIG. 44 is a sectional view of a conventional vertical power MOSFET.
As shown in FIG. 44, an n-type drift layer 112 is formed on one surface of an n-type semiconductor substrate 111 by epitaxial growth. P-type well layers 113 for MOS formation are selectively formed in the surface of the drift layer 112. N-type source layers 114 are selectively formed in the surfaces of the well layers 113. Trenches 115 are formed to reach the inside of the drift layer 112 from the surface of the source layers 114 through the well layers 113. Gate electrodes 119 are formed in the trenches 115 through silicon oxide films 118. In addition, a drain electrode 120 is formed on the other surface of the semiconductor substrate 111. Source electrodes 121 connected to the source layers 114 and well layers 113 are formed on the well layers 113.
Even in a case of ideal design, the characteristics of this type of power MOSFET are set in such a manner that the breakdown voltage and ON resistance must always satisfy the relationship defined by inequality (1). It has therefore been thought that any characteristics better than those defined by this relationship cannot be obtained.
Ron less than 2.2xc3x9710xe2x88x925Vb2.25xe2x80x83xe2x80x83(1)
where Vb is the static breakdown voltage, and Ron is the ON resistance.
However, it has recently been reported that the upper characteristic limit can be exceeded by burying a p-type diffusion layer in the drift layer 112. According to a structure having this buried diffusion layer, the ON resistance certainly decreases. However, since the junction distance (area) is long (large), the junction capacitance is large, resulting in slow switching. For the same reason, too many carriers are injected into a reverse-conducting diode incorporated in an element, and hence the element tends to break during a period of reverse recovery.
In practice, therefore, the range of application of elements having such structures is limited. In addition, in forming an element, many epitaxial layers are formed by repeating epitaxial growth and ion implantation, resulting in an increase in cost.
As described above, in a conventional power MOSFET, it is difficult to decrease the ON resistance. Even if the ON resistance can be decreased, the switching speed decreases and the characteristics of a reverse-conducting diode deteriorate. Furthermore, a problem arises in terms of cost.
According to the first aspect of the present invention, there is provided a semiconductor element comprising a semiconductor substrate of a first conductivity type having a first major surface and a second major surface opposing the first major surface, a drift layer of the first conductivity type formed on the first major surface of the semiconductor substrate, a well layer of a second conductivity type selectively formed in a surface of the drift layer, a source layer of the first conductivity type selectively formed in a surface of the well layer, a trench formed to reach at least an inside of the drift layer from the surface of the source layer through the well layer, a buried electrode formed in the trench through a first insulating film, a control electrode formed on the drift layer, the well layer, and the source layer through a second insulating film, a first main electrode formed on the second major surface of the semiconductor substrate, and a second main electrode connected to the source layer and the well layer.
According to the second aspect of the present invention, there is provided a semiconductor element comprising a semiconductor substrate of a first conductivity type having a first major surface and a second major surface opposing the first major surface, a drift layer of the first conductivity type formed on the first major surface of the semiconductor substrate, a well layer of a second conductivity type selectively formed in a surface of the drift layer, a source layer of the first conductivity type selectively formed in a surface of the well layer, a trench formed to reach at least an inside of the drift layer from the surface of the source layer through the well layer, a buried electrode formed through a first insulating film in a region extending from the trench of the drift layer to a bottom surface of the trench, a control electrode formed in a region extending from the source layer to the drift layer through the well layer in the trench to be insulated from the buried electrode through a second insulating film, a first main electrode formed on the second major surface of the semiconductor substrate, and a second main electrode connected to the source layer and the well layer.
According to the third aspect of the present invention, there is provided a semiconductor element comprising a semiconductor substrate of a first conductivity type having a first major surface and a second major surface opposing the first major surface, a drift layer of the first conductivity type formed on the first major surface of the semiconductor substrate, a trench formed to reach at least an inside of the drift layer from a surface of the drift layer, a buried electrode formed in the trench through a first insulating film, a well layer of a second conductivity type selectively formed in a surface of the drift layer between the trenches, a source layer of the first conductivity type selectively formed in a surface of the well layer, a control electrode formed on the drift layer, the well layer, and the source layer through a second insulating film, a first main electrode formed on the second major surface of the semiconductor substrate, and a second main electrode connected to the source layer and the well layer.
According to the fourth aspect of the present invention, there is provided a semiconductor element comprising a semiconductor substrate of a first conductivity type having a first major surface and a second major surface opposing the first major surface, a drift layer of the first conductivity type formed on the first major surface of the semiconductor substrate, a well layer of a second conductivity type selectively formed in a surface of the drift layer, a first trench formed to reach at least an inside of the drift layer through the well layer, a buried electrode formed in the first trench through a first insulating film, a source layer of the first conductivity type selectively formed in a surface of the well layer between the first trenches, a second trench formed to reach an inside of the drift layer from a surface of the source layer through the well layer, a control electrode formed in the second trench through a second insulating film, a first main electrode formed on the second major surface of the semiconductor substrate, and a second main electrode connected to the source layer and the well layer.
According to the fifth aspect of the present invention, there is provided a semiconductor element comprising a semiconductor substrate of a first conductivity type having a first major surface and a second major surface opposing the first major surface, a drift layer of the first conductivity type formed on the first major surface of the semiconductor substrate, a well layer of a second conductivity type selectively formed in a surface of the drift layer, a buried diffusion layer of the second conductivity type formed to reach at least an inside of the drift layer through the well layer, a source layer of the first conductivity type selectively formed in a surface of the well layer between the buried diffusion layers, a trench formed to reach an inside of the drift layer from a surface of the source layer through the well layer, a control electrode formed in the trench through an insulating film, a first main electrode formed on the second major surface of the semiconductor substrate, and a second main electrode connected to the source layer and the well layer.
According to the sixth aspect of the present invention, there is provided a semiconductor element comprising a semiconductor substrate of a first conductivity type having a first major surface and a second major surface opposing the first major surface, a drift layer of the first conductivity type formed on the first major surface of the semiconductor substrate, a buried diffusion layer of a second conductivity type formed to reach a portion near the semiconductor substrate from a surface of the drift layer, a well layer of the second conductivity type formed in the surface of the drift layer, a source layer of the first conductivity type selectively formed in a surface of the well layer between the buried diffusion layers, a trench formed to reach an inside of the drift layer from a surface of the source layer through the well layer and become shallower than the buried diffusion layer, a control electrode formed in the trench through an insulating film, a first main electrode formed on the second major surface of the semiconductor substrate, and a second main electrode connected to the source layer and the well layer.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.